The United States secretly has proposed to President Anastasio Somoza a plan for ending Nicaragua's bloody civil war, U.S. sources said yesterday. The plan calls for replacing Somoza by an appointed council that would try to form a government of national reconciliation.

The sources stressed that Washington does not know yet whether Somoza will accept the plan, which was relayed to him his week through Nicaraguan Foreign Minister Julio Quinana.

U.S. officials outlined the plan to Quintana following a meeting here of the Organization of American States last weekend, sources said.

They added that the new U.S. ambassador, Lawrence Pezzullo, who arrived yesterday at this post in Managua, is expected to receive Somoza's answer shortly.

As described by the sources, the U.S. plan is a four-stage process that would beging with Somoza resigning in favor of a constitutional successor and leaving the country.

His successor immediately would appoint a council or junta chosen from prominent Nicaraguans who have no ties with the Somoza government. As soon as that is done, the successor also would resign and turn the functions of government over to the council.

Then the council would attempt to mediate among Nicaragua's various anti-Somoza factions for a cease-fire and agreement on creation of a broadly based interim government. That government, in turn, would prepare the country for free elections and constitutional processes.

In order for the plan to work, the sources said, it would have to be accepted first by Somoza and, at a later stage, by the guerrillas of the Sandinista National Liberation Front fighting his regime.

The Sandinistas, whose leadership contains avowedly Marxist and proCuban elements, have denounced U.S. calls for a broadly based government as an attempt to cheat them of victory and have vowed to continue fighting until they attain power.

However, the sources said, the Carter administration is hopeful that if Somoza does resign under the conditions outlined in the U.S. plan, the Sandinistas might be induced through patient mediation to cooperate.

The sources stressed that the U.S. plan does not represent an attempt to set up a counter-force of non-Marxist elements that would freeze the Sandinistas out of any post-Somoza government.

There is general agreement within the U.S. government that such an attempt could not succeed, because of the strong support the Sandinistas have within the Nicaraguan populace and among democratic Latin American governments, the sources said.

Because of the Cuban connection, the administration is known to be fearful of the Sandinistas gaining complete control in Nicaragua. By calling for a broadly based government that would include Somoza opponents from the business and professional classes, the administration hopes to balance the Sandinistas with more moderate forces that could see the country through to an eventual free electoral choice of its future rulers.

The sources also stressed that, in offering the plan, the United States made clear that it has no intention of bargaining with Somoza about ways that would allow him to retain any measure of power or that would significantly alter the plan's main features.

That point was underscored publicly on Tuesday by Viron P. Vaky, assistant secretary of state for inter-American affairs. In testimony before a House subcommittee, Vaky said: "No negotiation, mediation or compromise can be achieved any longer with a Somoza government."

The sources said that Pezzullo, a career diplomat with extensive Latin American experience, had been instructed to inform Somoza that the United States will not deviate from Secretary of State Cyrus R. Vance's public call last week for a Nicaraguan government "which would be a clear break with the past."

Under Nicaraguan law, if Somoza resigned, his successor is supposed to be the president of the senate, Pablo Renner. However, the sources said, Washington is not particularly concerned about whom Somoza might choose since that individual would be in office only briefly as an instrument for appointing the national council.

But, the sources continued, the United States is insistent that the council members be free of any taint of association with Somoza. That, they said, is necessary to reassure the Nicaraguans that the plan is not a disguised attempt to propogate a system of what some Nicaraguans call "Somozaism without Somoza."

In announcing Pezzullo's departure from Washington yesterday, State Department spokesman Tom Reston said the ambassador had been instructed to refrain for the time being from presenting his diplomatic credentials to the Somoza government.

Somoza has called a special meeting of the Nicaraguan congress in Managua today, and reports from there last night said the capital was filled with rumors about what he might do. The U.S. sources said they doubt he will have made a decision about resigning by today.